Power line carrier frequency telephone system



July 7, 1942. G. sANDALLs, JR

r POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed June 19, 1941 3 Sheets-Sheet l /Nl/ENTOR G. SANDALLS, JR

July 7, 1942. G. sANDALLs, JR

POWER LINE CARRAIER FREQUENCY TELEPHONE SYSTEM Filed June 19, 1941 FIG? Auromr/c 200 TELEPHONE Exc/Nac 5 Sheets-Sheet 2 l /NVENTOR y G. SANDALL$,JR

July 7 1942- G. sANDALLs, JR 2,289,048

POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed June 19, 1941 3 Sheets-Sheet 5 AUTOMATIC TELEPHONE l EXCHANGE /NVE N TOR c. SANDALLS, JR.

. Bragg/a A 7` TORNE V Patented `luly 7, 1942 UNITED ASTATE s lPA'rENTf OFFICE PowEn LINE CARRIER FnEdUENoY TELEPHONE SYSTEM GeorgeSandalls, Jr., North Tarrytown, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New Application June 19, 1941, Serial No. 398,765

7 claims.

phone stations are enabled to connect with tele' phone stations outside of the power line system through an automatic Atelephone exchange or oice.

In accordance with this invention, a power line telephone system is arranged to include a plurality of telephone stations coupled to the power line and adapted to signal and to communicate directly with one another on high frequency or carrier waves over the power line. The power line telephone system is connected with an automatic telephone exchange 'or cnice through a suitable converting or translating circuit adapted to translate a signal pulse modulated carrier Wave into relay and/or switch actuating pulses, and to demodulate an audio frequency modulatedcarrier wave to provide the audio frequency currents for transmission over a conventional or voice frequency telephone system that correspond to the sound waves developed ata telephone station on the power line; and to modulate audio frequency current andvsignal pulses originating vin the voice frequency telephone system onto a high frequency or carrier wave for transmission to the power line telephone system. The translating or converting circuit for connecting the power line telephone system and the outside telephone system may be located at the automatic telephone exchange, or, in a suitable case, may be located at a point along the power line and be connected with the automatic telephone exchange over a suitable voice frequency path.

Telephone communication, that is, signaling and conversation, takes place on two high fre' quency electric Waves which. for convenience of description, will be denominated hereafter as carrier waves of high frequency Fi and F2. Calls made directly between telephone stations on the power line may be referred to as local or reverting calls. During such a reverting call, circuit conditions are established at the central office to prevent intrusion on the call from the central cnice. In making a `reverting call. the

calling party signals the called party in accordance with the preassigned signaling-code for the system on one of the high frequency waves, for example, on carrier wave Fl. Each station is normally in condition to receive such signals.

lAfter the called party has answered, the calling party transmits on the high frequency wave FI used for signaling and receives on the other high frequency, that is, on carrier wave F2. The

called party receives on the carrier wave FI and transmits on the other high frequency wave. Each station, of course, isprovided with a transmitting circuit and a receiving circuit for modulating the signal and voice frequencycurrents` on the high frequency wave used for transmitting and for demodulating the incoming modulated high frequency wave. In making a call through the automatic telephone exchange to a party not on the power line, the calling party establishes connection with the exchange by transmitting a signal pulse of high frequency, for example, carrier Wave F2. Upon hearing diai tone, which is transmitted to the calling station on the high frequency wave FI, the calling party operates his calling dial or A more complete understanding of the invention will be derived from the detailed description that follows taken in conjunction with the accompanying drawings. wherein:

Fig. 1 illustrates a power line carrier frequency i telephone system embodying the invention;

Fig. 2'V illustrates a modification of the telephone system of Fig. 1;

Fig. 3 illustrates a modification of the central y cnice arrangement of the telephone system of Fig. 1f; and c Fig. 4 illustrates in broad outline the nature of the telephone system embodying the invention shown in detail in Figs. 1, 2 and 3.

Fig. 1 shows a power line carrier frequency telephone system comprising a4 line |00. primarily adapted for the transmission and distribution of low frequency high voltage electric power and simultaneously used for high frequency or carrier wave telephony. A plurality of telephone stations A, two of which are shown and only one in detail, and a central oflice CO for thetelephone stations, are coupled to the power line. The latteris shown as a single phase line one of whose wires is grounded, but it may be, of course, an insulated single phase line, or comprise a pair of phase wires of a multiphase system or'other power line. Although only onetelephone station is shown in detail, it will be understood that each of the telephone stations is the same and operates in a manner similar to that to be specifically described. Fig. 4 illustrates in a. more general manner the arrangement contemplated by the invention. The telephone station OA represents a subscriber station of a telephone system served by the central oiiice CO, but not connected to the power line or included in the power line telephone system as are stations A. Connection may be established, however, between the stations A and OA through the central office as will be evident from what follows.

Description of power line telephone I station, Fig. 1 i

The telephone station A comprises a high frequency transmitting circuit I0, a high frequency receiving circuit II, a switching and filter circuit I2, and a coupling line I3. The transmitting circuit comprises a transmitter or microphone I4, a volume limiter or gain adjusting device i5, a modulator I6and an output amplifier I1, and terminates at the armatures I8, I9 of the circuit switching relay 20. The transmitter, which may be of the granular carbon type, is coupled to the device I by the transformer 2I and is connected in series with the transformer primary winding and a source 22 of talking current. The modulator I5 may be of any suitable type employing either electron discharge devices, or variable resistance devices such as copper-copper oxide' rectifier units. An oscillator 23 is associated with the modulator and is adapted to generate the high frequency or carrier waves FI and F2 for signaling and talking or transmitting purposes. The oscillator may comprise 'an electron discharge device circuit whose oscillation frequency determining circuit includes a pair of condensers 24, 25 connected, for example, in parallel. Condenser 25 is adapted to be removed from the circuit by the opening of the contact associated with the armature 26 of the relay 20. With condenser 24 only in the oscillator frequency determining circuit, the oscillation generated would be, for example, the high frequency FI. With both condensers in the circuit the wave generated would be the high frequency F2. The armature 21 and associated normally open lower contact of the relay or interrupter 28 are connected across the oscillator output. The interrupter 23 is adapted to vibrate at a suitable low frequency, for example, 20 cycles per second, and is provided with a lower armature 2S and associated normally closed lower contact connected through current source or battery 3| to ground. When the normally open key or pushbutton 32 is closed, an obvious circuit is closed for the energizing of the interrupter.

The receiving circuit is connected to the lower armatures 33, 34 of the relay 20, and comprises an amplifier 35, a demodulator 36, station signaling means 31, an attenuator network 38 and a telephone receiver 39. If desired, an automatic volume control connection 40 between the demodulator and amplifier' may be provided to maintain the demodulator output at a preassigned,level during receiving. The switchhook 4I is connected to the armature 42 of relay 20 and its associated normally open contact spring is connected to ground. The contact spring and switchhook are shown in engagement inasmuch as the receiver 39 is shown removed from the switchhook. A normally open key or push-button 43 is connected through the upper winding of relay to one terminal of the battery or other source of current 44. The other and lower winding of relay 20 is connected between the battery 44 .and the contact associated with the armature 42. A calling dial or impulse sending device 45, comprising a pair of pulsing springs 46, lis connected in the circuit between the oscillator 23 and the modulator I6 for a purpose to 'be brought out more in detail hereinafter.

Atacts associated with the armatures The circuit I2 is a part of the transmitting and receiving circuit and comprises a pair of bandpass filters 50, 60. The filter 50 is adapted selectively to transmit either or both side-bands of the modulated high frequency or carrier wave FI, and the iilter 60 is adapted selectively to transmit either or both side-bands of the modulated high frequency or carrier. wave F2. Each filter is connected between the coupling line I3 and a pair of normally closed contacts associated with the relay 2D, those of the filter 50 being the lower contacts associated with the armatures 33, 34 and those of the filter 60 being the upper con- I8, I9. Cross-connections 41 connect the filter 50 to the normally open lower contacts of armatures I8, I and cross-connections 48 connect the filter Suit the normally open upper contacts of armaturel 33, 34.

The coupling circuit I3 comprises a pair o1 conductors, one of which is connected directly to the grounded wire of the power line and the other of which is connected through a condenser 49 to thenon-grounded wire.4 If an insulated single phase power line or a pair of phase wires of a multiphase line are involved, a condenser should be connected in each coupling conductor. Instead of connecting the telephone station to the power line through a coupling condenser or condensers, the power distribution transformer located at the subscribers premises could be employed for coupling purposes. In any event, a suitable protector 5I should be included in the coupling circuit to protect the subscriber and the station equipment against the hazards of high voltage arising on the power line.

Call between two power line telephone stations Fig. 1

The operation of the system in the making of a local or reverting call, vthat is, a call between two telephone stations on the power line, will now be described. Prior to the initiation of the call, the receiver 39 will be on the hook 4I. Although not shown, it will be evident to the skilled in the art that additional switch contacts could be associated with the switchhook 4I and circuit connections provided so that, when the receiver is on the switchhook, the talking current circuit for transmitter I4 and the power supply circuit for the amplifiers I5 and I1 and the oscillator 23 would be interrupted, and so that, when the receiver is removed from the switchhook, the talking current circuit for `the transmitter I4 would be closed and also the circuit for the supply of power to the amplifiers l5 and I1 and the oscilr of the vacuum tube type. The receiving circuit amplifier 35, however, and the demodulator 33, if of the power consuming type, would be energized at all times so that the station would be in condition to receive incoming signaling pulses for operation of the station signaling means 31, and so that a party on the line could ascertain if the system was in use by placing his receiver to his ear while holding the -switchhook down. As shown in the drawings, the telephone station is in' condition for receiving on carrier wave F I' and for transmitting on carrier wave F2.

Let it be assumed that the power line is not being used `for telephonie transmission by another party or parties on the line and that the calling party is ready to initiate his call, that is, he has removed the receiver 39 from the hook 4I. Closure of the key 43 completes an obvious circuit for the upper winding of relay 20 which locks up through its lower winding and amature 42. The lter 50 is connected through the connections 41 to the armatures I3, I3 and. hence, to the transmitting circuit I0, and the lter 30 is connected through connections 48 to the armatures 33, 34 and, hence, to the receiving circuit II.

The armature 26 is disengaged from its associated contact to remove condenser 25 from the oscillators frequency determiningcircuit. The oscillator delivers a high frequency FI to the modulator and through the lter 50, to the power line.

The calling party then depresses or pushes key 32 the number of times and in accordance with the code prescribed for the power line telephone system to signal .the desired station to operate the interrupter 28. This causes armature 21 to vibratefin association with its normally open contact to modulate the carrier wave supplied to the modulator. The carrier wave is modulated at the signaling rate, is transmitted through the filter 50 to the power line kand passes over the latter to all ofthe stations. At each telephone station, the incoming modulated carrier wave FI is transmitted by the filter 50 to the receiving circuit, is amplified in amplier 35, demodulated by demodulator 38 'andoperates the station sighas terminated, the replacing of the receiver on the switchhook results in the deenergization of the relay 20 as a result of the removal of ground at the switchhook and the restoration of the station circuit to its original condition in which it is adapted to receive on the carrier wave FI.

Description of power line system centralofice,

' Fia. 1

Before describing the manner in which a call may be made from a telephone station on the power line telephone system to a telephone station of another or a Ageneral telephone system through an automatic exchange or central office, the circuit arrangement and equipment at the central oilice CO will be described. The central office circuit functions in general to translate or convert carrier current supervision and dialing into common battery signals suitable for actuating any conventional step-by-step, crossbar or all-relay dial equipment indicated generally at 200, and, also, to demodulatethe modulated carrier wave incoming from the power line and to transmit a modulated carrier wave to the power line.

The central ofdce comprises a transmitting circuit llil and a receiving circuit II' coupled to the power line through a coupling circuit I3' andv coupled to the machine switching equipment and circuit 200 through the hybrid coil 10. The receiving circuit comprises a band-pass filter 5B', adapted to transmit one or both side-bands of the modulated carrier wave F2 originating ata tele# phone station A, an amplifier 12. and a demodulator 13. An automatic volume control circuit 14- may be included to maintain constant the level naling means 31. .In answering, the called party removes his receiver from its switchhook and the called station circuit is the same asrwas that of the calling station before the key 43 was depressed, that is, the called station is arranged to transmit on the carrier wave. F2 and tov receive' on the carrier wave FI. When the called party answers, the calling party talks into the transmitter I4, the audio frequency currents correspending to the sound waves acting on the transmitter that'are developed in the transformer 2| are suitably amplified in the device I5 and transmitted to the modulator, to modulate the carrier wave FI in accordance therewith. The modulated carrier wave passes through the output amplifier I1 and the lter 50 to the power line and over the latter to the called station. Since 'thecalled staof the signals at the output of the demodulator. The output of the demodulator 13 is connected with the lower winding 1I of the hybrid coil. The transmitting circuit comprises -awmodulator 15, a band-pass nlter 55'.. adapted to transmit one or both side-bands of the modulated carrier wave FI, and an output amplifier 15. The modulator input is connected with the upper winding vided for Purposes that will become apparent from the further description hereinafter.v A

common source 13 of enersizns current for relays v B, C, E G is included in the central oiiice..`

5 canv between power une telephone 'station and the centrall omce, Fia. 1

f Let itbe assumed that the subscriber at a. station A on the power line wishes to make' a call 00 Ythrough the central cnice. As in the'case oi' a retion transmits on the carrier wave F2, transmission incoming to the calling station will comprise 55 power line through filter 50. The carrier wave network 33 is provided to equalize for the lower transmission level required for listening compared to that required for operation of the staverting call .or calllocal to the power line, removai of the receiver from its hook energizes the amplifiers and oscillator in the transmitting circuit, and the carrier wave F2 is applied tothe is transmitted by the filter at the central office Closure ofcontact set I of relay H completes an 10` obvious energizing circuit for relay B whose'asl sociated contact set closes. Closure 'of ,contact t set 2 of relay H places ground on? sleeve lead-'S causing the line terminal to test busy so long as the-calling party maintainsv the connection with tion 'signaling means 31. When the conversation 75 the central omce. Closure of contact set 3 of 11 of the hybrid coil and, also, with an oscillator relay H through hybrid coil winding 80 prepares a metallic circuit between the tip and ring conductors T, R of the line circuit. After removing the receiver 3S from its hook, the calling party operates his signaling key 32 to operate the interrupter 28. The carrier wave F2 is modulated at a low frequency rate. The modulated carrier wave is received at the central ofiice, is amplified and demodulated, and the low frequency demodulation component energizes the relay C. The associated normally open contact sets I, 2 of relay C close. Relay C locks up over a circuit comprising ground, current source 19, lower winding and contact set I of relay C, contact set of relay B, to ground. Closure of contact set 2 of relay C completes the metallic circuit between the tip and ring conductors T, R causing the line relay tov operate and the line finder to seize the line. Closure of contact set I of relay C also completes an energizing circuit for relay G: ground, source 19, relay G, contact set I of relay C, contact set of relay B, ground. This removes the short circuit on the oscillator 18 whereby the carrier wave FI is applied to the power line through the modulator, filter 50 and amplifier 16. This carrier wave is modulated by dial tone from the first selector or link supplied to the modulator through the hybrid coil.

Upon hearing the dial tone, the calling party releases the signaling key 32, and proceeds to operate his calling dial '45 in well-known manner, interrupting the oscillator output as a result of the break-and-make of impulse springs I6 and thereby modulating the carrier wave F2 in accordance with the dial pulses. Relay H follows the dial pulses, releasing and momentarily inter- Y rupting the line circuit loop for each break of the dial impulse springs. The desired connection is established with the outside called party through the pulse operated switches of circuit 200 in well-known manner. At the conclusion of the mally closed contact set of relay G opens to remove the short circuit on oscillator 18 and carrier wave FI is delivered to the oscillator; operation of relay C closes the latters normally open contact set 2 to complete the tip and ring conringing current supplied to it through the hybrid coil, and the carrier wave from the oscillator 18 is interrupted in accordance with such signaling current input to relay F. The modulated carrier wave is transmitted to the power line and over the latter to all the stations connected to it. The modulated wave is transmitted by the filter 50 and amplifier 35, and the demodulated outgoing call, when the calling party disconnects Y by restoring his receiver to its hook, the relays H, B. C and G 'release and the central ofce circuit Ais restored to'its original condition.

In carrying on'conversation with the called party, the power line telephone subscriber transmits on the carrier wave F2 and receives on thev n carrier wave FI. At the central office, `the modulated carrier wave F2 is demodulated, and the y audio frequency components corresponding to the sound waves developed at the power line subscribers transmitter I4, are transmitted through the windings 1I and 80 ofthe hybrid coil and the tip and ring conductors to the called party. The audio frequency currents originating with the called party and incoming'to the central omce are transmitted overthe tip and ring conductors and through the hybrid coil windings 80 and 11 to the modulator 15 in which they are modulated on the carrier waveFI. The modulated carrier wave is transmitted to the power line and over the latter to the station A, and through the filter il and receiving circuit II thereat to the subscribers demodulating equipment and receiver.

On a call incoming to the central ofce for a telephone station on the power line, the operation is as follows: Ringing .current from the link or connector energizes relay D. Closure of its normally open contact set completes an obvious circuit for the upper winding of relay E. The latter, through its lower winding and its contact let I, locks up to the .sleeve conductor S and completes, through its contact set 2, obvious energidng circuits for relays G and C. The norwave is delivered to the station signal 31. The particular subscriber'whose code has been signaled answers by removing his receiver and transmitting on carrier wave F2 and receiving on carrier wave FI. i

It was assumed in the situation described in the paragraph immediately above that the power line was not in use, i. e., that two of the teley phone stations coupled to it were not carrying on a conversation. That the 'power line was busy" would be indicated, however, in the following manner. It has already been pointed out that, on a reverting call or call local to the power line system, the calling party transmits on the carrier wave FI and receives on the carrier wave F2, and that the called partytransmlts on the carrier wave F2 and receives on the carrier wave FI. Since each telephone station A is arranged in its normal condition to receive on the carrier wave FI, and signaling from one station A to another on the power line is on the carrier wave FI, signaling between the stations has no effect on the central .oiiice inasmuch as the receiving circuit I I' contains the band-pass filter 60 that' transmits only the carrier wave F2 and one or more of its side-bands. When the called party answers, however, the called party's oscillator supplies the carrier wave F2 to the line, and transmitting out of the called station is by modulating the carrier wave F2. As soon as the carrier wave F2 appears on the power line, relay H is operated; i. e., the carrier wave is transmitted by filter 60 to the amplifier 12 and demodulator 13 and energizes relay H, The normally open contact sets of the relay H close and its contact set 2 connects the sleeve conductor S to ground causing the line terminal to test busy at the connector terminal in equipment 200 until the called party disconnects.

Description of modified central office. Fig. 3

It will be observed from the above description of circuit operation that every reverting call involves an unguarded interval: revertlve ringing or signaling by the calling subscriber does not cause the Vline connected to the power line through the carrier frequency equipment and relay arrangement at the' central oiiice to test busy at the connector terminal. The calling party signals on carrier wave FI, whereas, the central oice receives the carrier wave F2 only. The line is not guarded or rendered "busy until the called subscriber answers and places carrier wave F2 on the power line. Fig. 3 shows a circuit arrangement that is al modification of the central omce of Fig. 1, and that overcomes this condition. The central oiiice CO' is the same as that of the cen- 'tral oiiice C0 of Fig. 1 except that relay G ining carrier wave FI is connected to the receiving circuit I'i'. An amplifier-Stand demodulator or detector 92, provided with an A. V. C. connection 93, are connected to the output of the lter, a

relay K being connected in the output circuit ofA the demodulator. As soon as carrier wave Fl is applied to the power line by a calling subscriber than the power line telephone system. When q there is transmission on carrier wave FI out of the central oi'ce to the power line, relay K might be energized, but the outgoing transmission is not affected.

Description of power line telephone system of Fig. 2

The arrangement described with reference to Figs. 1 and 3 requires three conductors (tip, ring, sleeve) between the dial equipment 200 and the carrier terminal or convertor equipment at the central oice CO or CO'. In the system of Fig. 2 only two conductors (tip and ring) are required between the carrier terminal equipment and the circuitzlil). With the arrangement of Fig. 2, the carrier frequency equipment interconnecting the high frequency and the low frequency portions of the system can be located at any convenient point, along the power line. and be connected with the machine switching equipment 200 at a point remote from the power line over an ordinary two-conductor metallic line.

The'telephone station VA of Fig. 2 is the same as station A of Fig. 1 except that the key or button 43 is omitted, and the key 32' serves the dual function of completing an energizing circuit for relay 20, and of completing the energizing circuit for the interrupter28. The central ing and associated ycontact set connectedy to ground through the switchhook 4I. The oscillator output is shifted to carrier wave FI, and, with the interchange of filters 50, B0 between the transmitting and receiving circuits l0, Il, carrier wave FI is transmitted tothe power line through filter 50 and the station is adapted to receive on carrier wave F2. 'Operation of key 32 in accordance with the signaling code for the power line telephone system modulates the carrier wave FI in accordance therewith. The modulated carrierwave is transmitted to all of the other telephone stations on the line and, since each station is adapted normally to receive on carrier wave FI the station signal 31 at each station is operated by the Vdemodulated output of its demodulator 36. The called station answers v by lthe removal of its receiver from its hook,

and the beginning of the conversation by transmission on carrier wave F2 since the relay 20- of the called station is not energized. When the call is terminated, the called and calling parties replace their receivers' on the switchhooks at the oflice CO2 is similar to the central oiiice CO of Fig.- 1 in that it includes a carrier frequency transmitting circuit I0', a carrier frequency receiving circuit il', a hybrid coil 1|!" for inter- -connecting the audio-frequency circuit of the central oiiice with the carrier frequency circuits connected to the power line, a circuit 95 for causing the power line to test "busy" at the central oiiice during the period a reverting cali .is

. being completed, and a plurality of relays KI HI and DI for supervising a connection. between 'a station on the-power line and the central omce and for establishing appropriate signaling and supervisory conditions at thecentral ofce during either a reverting call or one completed through the central oiilce. The number of relays required at the central ofiice in the system of Fig. 2 is less than inthat of Fig. 1. The contact sets associated' with the relays are shown in their` condition of rest, i. e., when the power line telephone system is not in use. The operation of the stations.

When the calling party initiatesf thelcall by removing his receiver from its hook, there is a` short interval before he operates the key 32' during which carrier wave F2 is supplied to the power line. This carrier wave incoming to the central ofllce is transmitted by the filter to the amplier and demodulator of the receiving circuit VI I'. The relay HI, connected in the demodulatin circuit, is energized, and closure of its contact set I closes the loop between conductors T, R through the hybrid coil, and causes the line .to testbusy at its connector terminal:

the carrier frequency circuit is thus protected from intrusion from the central ofce. After the calling station startssignaling by operation of key 32' and, thus,I is transmitting on the carrier wave FI relay HI releases, but relay KI operates. The modulated carrier wave FI is notonly transmitted to the other stations on the power line, but is received at thecentral oiiice through the circuit 95. Filter SII' transmits the modulated wave'to amplifier 9|' and demodulator 82' in whose output circuit the relay Ki is connected.

Energlzation of relay Kl opens its normally closed contact set I disconnecting ground from.A

relay HI, and closes its -normally open contact set 2, thus placing a short circuit on the conductors T, R and causing the line to test busy 4 at the connector terminal.l While relay KI is operated and relay Hi is released, the normally open contact set 3 of relay Hl remains, open;

thel oscillator 'I8' remains unoperated. It is as.-

' sumed that the line circuit of the equipment 200 systemof Fig. 2 with respect to a call local to power line stations, and a. call completed throughY the central omce to or from one of the power line telephone stations will now be described.

' Call between power line telephone stations, Fig. 2

to the power line. Closure of key 32'l completes I an energizingcircuit for the upper winding. of relay 20, which locks up through its lower wind- `isof the lock-out type, -such as is commonly employed in community dial oiiices; if it is not,

a link or line finder and mst selector `wiil be held up during'the reverting call.

' As already describedthe audio frequency line loop at4 the central ofiice is closed initially" by the operation of relay HI and, when the calling station shifts from carrier wave F2 to F|,"this closure is replaced by the operation of relay KI. Relay HI, however, must be able toV function as a pulse repeating relay on calls from a station A' to the central oice CO2; this requires that it be of the fast-release type. Relay Kl, therefore, should-be of a very fast-operating type so 'that any pulse generated between the release of relay HI and the energizing ofv relay KI will be of insumcient duration to cause false operation of the dial equipment at thetelephone exchange.

Call between power line telephone station and central oice, Fig. 2

KI. Dial tone from equipment 200. supplied overthe conductors T, R and through the lower winding of the hybrid coil, is modulated on the carrier wave FI, in modulator and transmitted through filter 50 and amplier 16 to the power line and over the latter to the calling station.

At the latter, the dial tone modulated carrier wave is demodulated and dial tone appears in the receiver. 'I'he calling party does not operate his key 32', hence the station transmits on carrier wave F2 and receives on carrier wave Fl.

The calling party operates his dial 45 in the conf ventional manner and the break-and-make of impulse springs 46 modulate the carrier wave F2 in accordance therewith. The pulse modulated wave is received at the central ofce CO2 and the relay HI repeats the dial pulses into the central omce dial equipment. When the connection to the called party is complete, conversation between the calling station A' and the calling party proceeds as `in the case of the system of Fig. 1.

When a call is incoming to a station of the power line telephone system from the central oiiice, the operation of the system of Fig. 2 is as follows. When the line comprising conductors T, R is seized, relay DI follows the ringing current supplied by the connector. 'I'his relay is tuned to a low frequency, e. g., C. P. S., and closes its contact set I and opens its contact set 2 at that rate whereby the oscillator 18' has energizing potential connected to it twenty times a second during each ringing period; accordingly, carrier wave FI is applied to the power line at that rate and. 'transmitted to all stations A'..

When the station AN whose'code has been used answers, carrier wave F2 originating at the station causes relay HI at the central oflice to operate, closing the loop for conductors T, R through the hybrid coil over contact set 1 of relay Hl, and energizing the oscillator 18' over contact set 3 of relay HI. When the call is terminated and the parties restore their receivers to their supports, the circuit of the central office CO2 restores to the condition shown .on the drawing.

Although this invention has been disclosed with reference to certain specific embodiments, it will be understood that it is not limited thereto, but

is of a scope evidenced by the appended claims.

What is claimed is:

1. In a power line carrier frequency telephone system comprising a power line, a plurality of carrier frequency telephone stations coupled to said line and adapted to establish direct connection with one another over said line, and a central omce including means adapted to be actuated by carrier frequency pulses originating at a power line telephone station for connecting the'latter with a telephone station of another telephone system, and, in which, after a connection is established between two of the telephone stations coupled to the power line, one of said stations transmits and receives on a first and a second carrier wave, respectively, andthe other of said stations transmits and receives on said second and first carrier waves, respectively; means at said central office, responsive to each of said carrier waves at diiferent stages of establishment of the connection between the power line telephone stations to prevent a call incoming to the power line from the central oiiice interfering with the connection being established on the power line.

2. A power line telephone system comprising a power line, a high frequency telephone station connected to said line, means at said station for generating a plurality of high frequency electric waves, a circuit for connecting the power line to an automatic telephone 'exchange through which a telephone station coupled to the power line may be connected' to a telephone station not coupled to the power line, said connecting circuit including an audio frequency line for connecting it with the automatic exchange, means at said power line station ,for transmitting a pulse of one of said high frequency waves over said power line to said connecting circuit, means in said connecting circuit responsive to said pulse to cause said audio frequency line to test busy at said automatic exchange during said pulse, means in said power line station to prevent further transmission of said one high frequency wave and to transmit pulses on said second high frequency wave over said power line to said connecting circuit, and means in said connecting circuit responsive to the pulses of said second high frequency wave to transmit corresponding pulsesover said audio frequency line to the automatic exchange.

3. A power line telephone system comprising a power line, a plurality of telephone stations coupled to said power line, a circuit for connecting the power line with an automatic telephone exchange whereby said telephone station may be connected through said automatic exchange with telephone stations that are not coupled to the power line, said connecting circuit including an audio frequency line for connecting it with the automatic exchange, each telephone station connected to the power line including means to generate two different high frequency electric waves, means` at each of said power line stations for signaling to another power line station on one of said high frequency waves, means in said connecting circuit responsive to said one high frequency wave to cause the audio frequency line to test busy at the exchange during said signaling, means at each power line station for responding to a calling signal from another of said power line stations by transmitting to the 'calling sta' tion on the second of said high frequency waves, and means in said connecting circuit responsive to said second high frequency wave to cause the audio frequency line to test busy at the automatic exchange during the remainder of a connection between the power line telephone stations.

4. A power line telephone system comprising a power line, a plurality of high frequency telephone stations connected to said line, means at each of said stations for generating a plurality of high frequency waves, a circuit for connecting the power line to an automatic telephone exchange, said circuit including an audio frequency line, means-at each power line station for transmitting and receiving on each of said high frequency waves, each station normally being in condition to receive on the same one of said high frequency waves and being adapted to transmit initially on a second of said high frequency waves, switching means at each station to enable it to transmit on said one wave and to receive on said second wave, said switching means being operated when it is desired to establish a connection with a second station on the power line, means in said connecting circuit responsive to said second wave transmitted to the power line from one of said 'stations in initiating a connection 'with a second of said stations, prior to operation of the switching means at said one station, for causing said audio frequency line to testbusy at the exchange, and additional means in said connecting circuit responsive to said first wave after said switching means has been operated, to maintain the busy condition of said audio frequency line until said second station responds to signaling from said one station on said first wave and transmits on said second Wave.

5. Atelephone system comprising a transmission line, a plurality of carrier frequency telephone stations coupled to said line, said stations being adapted to signal and to communicate directly with each other over said line on two carrier frequency waves, and a circuit for connecting said transmission line with an automatic telephone exchange, said circuit including an audio frequency line connecting it with the exchange, a carrier frequency line connecting the circuit with said transmission line and a carrier frequency receiving circuit, said receiving circuit including a plurality of networks selective to a plurality of preassigned frequency bands, one band including one of said carrier waves and another including the second of said carrier waves. a demodulator connected to the output of each network, and a plurality of relays each responsive to the output of a different one of said demodulators for rendering said audio frequency line busy to a call incoming to the telephone system from the exchange when either carrier wave is applied to the transmission line during the period of a connection between stations on the transmission line.

6. A telephone system comprising a transmission line, a plurality of telephone stations coupled to said line, a circuit for connecting the line with a telephone exchange whereby said telephone station may be connected through vsaid exchange with a telephone station not coupled to the transmission line, said connecting circuit including an audio frequency line for connecting it with the exchange, each transmission line station including means to generate two different high frequency electric waves, meansl at each of said transmission line stations for signaling to another transmission line station on one of said high frequency Waves, means in said connecting circuit responsive to said one high frequency Wave to cause the audio frequency line to test busy at the exchange during such signaling, means at each transmission line station for responding to a calling signal from another of said transmission line stations by transmitting to the calling station on the second of said high frequency waves, and means in said connecting circuit responsive to said second high frequency wave to cause the audio .frequency line to test busy at the exchange during the remainder of a connection between the transmission line stations.

7. A telephone system comprising a transmission line, a plurality of carrier frequency telephone stations coupled to said line, said stations being arranged to establish and maintain a connection directly with each other over said line,

each station, when the calling station, signalingl to the called station on a first carrier wave, transmitting on the same carrier wave and receiving on a lsecond carrier wave, and each station, when the called station, transmitting to the calling stationon said second carrier wave and receiving from the calling station on said first carrier wave, a circuit connected to said line to enable each of said stations to communicate with a telephone station not coupled to said line, said circuit including an audio frequency line leadling to an exchange for said last-mentioned station,- each transmission line circuit when communicating through the connecting circuit signaling and transmitting on said second carrier wave and receiving on said first carrier wave, said connecting circuit including a receiving circuit to receive on vsaid second carrier wave and a transmitting circuit to transmit on said rst farrier wave, means in said connecting circuit responsive to said first carrier wave when a transmission line station is signaling to a second transmissionY line station to cause said audio frequency line to test busy at the exchange, and means in said receiving circuit responsive to said second carrier wave when a transmission line station is connected through the connecting circuit with the exchange, to disable said last-mentioned means against operation by said first carrier wave as transmission from said connecting circuit to said transmission line takes place on said first carrier wave.

GEORGE SANDALLS, JR. 

